The human eye is an optical system which employs a lens to focus light rays representing images onto the retina within the eye. The sharpness of the images produced on the retina is a factor in determining the visual acuity of the eye. Imperfections within the lens and other components and material within the eye, however, may cause the light rays to deviate from a desired path. These deviations, referred to as aberrations, result in blurred images and decreased visual acuity. Hence, a method and apparatus for measuring aberrations is desirable to aid in the correction of such problems.
One method of detecting aberrations introduced by an eye involves determining the aberrations of light rays exiting from within the eye. A beam of light directed into the eye as a point on the retina is reflected or scattered back out of the eye as a wavefront. The wavefront represents the direction of light rays exiting from the eye. By determining the propagation direction of individual portions of the wavefront, the aberrations introduced to the light rays passing through parts of the eye such as the cornea can be determined and corrected. In this type of system, increased accuracy in determining the aberrations can be achieved by reducing the size of the regions of the wavefront used to derive the propagation direction.
A general illustration of the generation of a wavefront is shown in FIG. 1. FIG. 1 is a schematic view of a wavefront 10 generated by reflecting a laser beam 12 off of the retina 20 of an eye 16. The laser beam 12 focuses to a small spot 14 on the retina 20. The retina 20, acting as a diffuse reflector, reflects the laser beam 12, resulting in a point source wavefront 10. Ideally, the wavefront 10 from a point source leaving a perfect eye would be represented by a spherical or planar wavefront 22. However, aberrations introduced by the eye 16 as the wavefront passes out of the eye result in an imperfect wavefront, as illustrated by the wavefront 10. The wavefront 10 represents aberrations which lead to defocus, astigmatism, spherical aberrations, coma, and other irregularities. Measuring and correcting these aberrations allow the eye 16 to approach its full potential, i.e., the limits of visual resolution.
FIG. 2 is an illustration of a prior art apparatus for measuring the wavefront 10 as illustrated in FIG. 1. By measuring the aberrations, corrective lens can be produced and/or corrective procedures performed to improve vision. In FIG. 2, a laser 22 generates the laser beam 12 which is routed to the eye 16 by a beam splitter 25. The laser beam 12 forms a spot 14 on the retina 20 of the eye 16. The retina reflects the light from the spot 14 to create a point source wavefront 10 which becomes aberrated as it passes through the lens and other components and material within the eye 16. The wavefront 10 passes through the beam splitter 25 toward a wavefront sensor 26. The apparatus described in FIG. 2 is commonly described as single-pass wavefront measurement system.
Typical prior art wavefront sensors 26 include either an aberroscope 30 and an imaging plane 28, as illustrated in FIG. 3, or a Hartmann-Shack sensor 40 and an imaging plane 28, as illustrated in FIG. 4. The wavefront sensor 26 samples the wavefront 10 by passing the wavefront 10 through the aberroscope 30 or the Hartmann-Shack sensor 40, resulting in the wavefront 10 producing an array of spots on an imaging plane 28. Generally, the imaging plane 28 is a charge coupled device (CCD) camera. By comparing an array of spots produced by a reference wavefront to the array of spots produced by the wavefront 10, the aberrations introduced by the eye 16 can be computed.
Each spot on the imaging plane 28 represents a portion of the wavefront 10, with smaller portions enabling the aberrations to be determined with greater precision. Thus, the smaller the sub-aperture spacing 32 and the size of the sub-aperture 33 in the aberroscope 30 of FIG. 3, and the smaller the lenslet sub-aperture spacing 42 in the Hartmann-Shack sensor 40 of FIG. 4, the more accurately the aberrations can be determined.
An example of a Hartmann-Shack system is described in U.S. Pat. No. 6,095,651 to Williams et al., entitled Method and Apparatus for Improving Vision and the Resolution of Retinal Images, filed on Jul. 2, 1999, incorporated herein by reference.
The resolution of the aberrations in such prior art devices, however, is limited by the grid size 32 and aperture size 33 in an aberroscope 30 (see FIG. 3), and by the lenslet sub-aperture spacing 42 in a Hartmann-Shack sensor 40 (see FIG. 4). Due to foldover, reductions to grid size 32 and lenslet sub-aperture spacing 42 are limited. Foldover occurs in an aberroscope sensor 30, for example, when two or more spots 31A, 31B, and 31C on imaging plane 28 overlap thereby leading to confusion between adjacent sub-aperture spots. Similarly, foldover occurs in Hartmann-Shack sensors 40 when two or more spots 41A, 41B, 41C, and 41D on imaging plane 28 overlap. Foldover may result from a grid size 32 or lenslet sub-aperture spacing 42 which is too small, a high degree of aberration, or a combination of these conditions. Hence, the grid size 32 or lenslet sub-aperture spacing 42 must be balanced to achieve good spatial resolution while enabling the measurement of large aberrations. Accordingly, the ability to measure a high degree of aberration comes at the expense of spatial resolution and vice versa.
The constraints imposed by the aberroscope and Hartmann-Shack approaches limit the effectiveness of these systems for measuring large aberrations with a high degree of spatial resolution. These limitations prevent optical systems with large aberrations from being measured, thereby preventing them from achieving their full potential. Accordingly, ophthalmic devices and methods which can measure a wide range of aberrations with a high degree of spatial resolution would be useful.